Tire Comprising An Improved Tread

ABSTRACT

Tire tread has a central rib ( 411 - 413 ) and two lateral ribs ( 421 - 422 ) which are separated by circumferential grooves ( 141 - 144 ). A first lateral rib ( 421 ) adjacent to the outer axial edge of the tread and a central rib have a first portion ( 4211, 4111, 4121, 4131 ) the axial width of which exceeds 20% of the axial width of the relevant rib made from a first rubber composition containing an elastomer and a reinforcing filler consisting predominantly of carbon black, and an axially adjacent second portion ( 4212, 4112, 4122, 4132 ) made of a second rubber composition containing an elastomer and a reinforcing filler a minority proportion of which consists of carbon black. The second rubber composition has a tan d0 value which is less than the tan d0 value of the first rubber composition, the tan d0 values being measured at a temperature of 0 C and under a stress of 0.7 MPa.

FIELD OF THE INVENTION

The present invention relates to tires for passenger vehicles. Itrelates more particularly to tires suited to a sporty on-road drivingstyle.

BACKGROUND

The grip of the tires on the ground on which they are running is, from adriver safety viewpoint, one of the most important features of a vehiclethat is fitted with tires. It is also what governs the performance ofthe vehicle in sporty on-road driving: if its tires lose their abilityto steer as a result of a lack of grip, the vehicle can no longer besteered.

Of course, a vehicle, even if intended for sporting use, has to driveunder variable weather conditions. It is therefore known practice toprovide the tire with means that provide good grip on dry ground and onwet ground. Notably it is possible to adapt at least part of the treadpattern to use on wet ground, for example by providing voids able tostore water and/or allow water to drain away, or by increasing thenumber of edge corners on the tread pattern so as to cut through thefilm of water formed between the tread and the ground. It is alsopossible to vary the materials of which the tread is made, by usingrubber compositions that are more particularly suited to use on wetground and/or on dry ground. A tread having both types of rubbercomposition is able to provide good grip under all circumstances. Anexample of such a tire is given in document EP 1 308 319.

During sporty on-road driving, the tires of a vehicle experience hightransverse loadings when the vehicle fitted with these tires iscornering. During the corner, the transverse loadings cause the contactpatch in which each tire makes contact with the ground on which it isrunning, to undergo deformation that can be likened to skewing into atrapezium shape: the side of the contact patch that is furthest from thecentre of the corner lengthens, whereas the side of the contact patchwhich is closest to the centre of the corner shortens.

The “side of the contact patch that is furthest from the centre of thecorner” is the side via which, in the direction of the rate of drift ofthe centre of the wheel on which the tire is mounted, the elements ofthe tread come into contact with the ground. For this reason, it issometimes referred to as the (transverse) leading edge. The oppositeside, namely the “side of the contact patch closest to the centre of thecorner” is sometimes referred to as the (transverse) trailing edge.

This “trapezoidal skew” deformation alters both the load borne by thevarious ribs of the tread and the contribution that each makes to thetransverse loading developed by the tire. For a given load, that one ofthe tires of the vehicle has to bear in a given cornering rate, the ribswhich have lengthened bear a greater share of the total load borne bythe tire. The ribs that have shortened bear a correspondingly smallerproportion of the total load borne by the tire. For a given transverseloading, delivered by one of the tires under given cornering conditions,the result is that the most heavily loaded ribs (which in general meansthose on the side furthest from the centre of the corner) are thosewhich also make the greatest contribution towards the total transverseloading.

Rubber compositions suited to use on wet ground are generally moresensitive to the very high thermal and mechanical stresses generated inthe contact patch of a tire under severe cornering conditions on a dryroad surface. If the tread of the tire is provided with portions made ofa rubber composition that has better grip on dry ground and withportions made of rubber composition that has better grip on wet ground,it is preferable to ensure that the rubber composition that has bettergrip on dry ground is positioned on that side of the contact patch whichis furthest away from the centre of the corner. Thus, even if thecontact patch is skewed into a trapezium shape, the tire will maintaingood grip on dry ground, which means to say that it will maintain a goodability to develop a high transverse loading. Furthermore, because theground contact pressures are highest on this same side of the contactpatch (which is the side furthest away from the centre of the corner),it is this part of the contact patch that generally provides the bestdrainage of the water wetting the road surface. Consequently, thisregion of the tread lends itself well to the use of a rubber compositionthat exhibits better grip on dry ground. In other words, in this region,the tire drives as if it were driving along dry ground. There istherefore no benefit in making this part of the tread from a rubbercomposition that exhibits better grip on wet ground but of which theperformance on dry ground is inferior to that of a rubber compositionthat exhibits better grip on dry ground. The “Pilot Sport 2” tire soldby Michelin is one example of a tire in the tread of which the rubbercompositions are arranged in this manner.

Despite the good performance exhibited by this tire in terms of grip,there is still an increasing need to improve the compromise between drygrip and wet grip of tires and, more particularly, tires designed forsporty on-road driving. This is why the applicant company, in its patentapplications WO 2011/076680 A1 and WO 2012/175444 A1, has proposeddividing the tread into several axis zones and distributing the rubbercompositions with better wet grip and the rubber compositions withbetter dry grip carefully across these zones. While these solutions havemade it possible to obtain a better compromise between dry grip and wetgrip, they are not, however, optimal in terms of grip on wet ground.

SUMMARY OF THE INVENTION

One of the objectives of the present invention is therefore to provide atire that provides a better compromise between wet grip and dry grip.

This objective is achieved by a tire which has a predetermined directionof mounting, in which a portion of that part of the tread that istraditionally reserved for the rubber compositions that have better gripon wet ground is made from a rubber composition that has better grip ondry ground, and a portion of the part of the tread that is traditionallyreserved for rubber compositions that have better grip on dry ground ismade of a rubber composition that has better grip on wet ground.

More specifically, the objective is achieved by a tire intended to bemounted on a mounting rim of a wheel of a vehicle and having apredetermined direction of mounting on the vehicle, comprising a treadextending between an outer axial edge and an inner axial edge, the inneraxial edge being the edge intended to be mounted on the vehicle bodyshell side when the tire is mounted on the vehicle in the saidpredetermined direction of mounting, the axial distance between theouter axial edge and the inner axial edge defining an axial width L ofthe tread. The tread comprises, in any radial section, at least onecentral rib and two lateral ribs, the lateral ribs being positioned oneon each side of the said at least one central rib, the ribs beingseparated by circumferential grooves each having two lateral walls, themean axial distance between the two lateral walls of each groove beingpreferably greater than or equal to 5 mm and less than or equal to 20mm. Each central rib is axially delimited by an outer boundary and by aninner boundary, each boundary being formed by a lateral wall of acircumferential groove, the inner boundary being axially closer to theinner axial edge of the tread than the outer boundary, the axialdistance, on the tread surface in the new state (i.e. not worn), betweenthe outer boundary and the inner boundary defining an axial width LC ofthe central rib. A first lateral rib is adjacent to the outer axial edgeof the tread and delimited axially by an outer boundary and an innerboundary, the outer boundary being situated, over the entire radialdepth of the first lateral rib, an axial distance DE1 from the inneraxial edge of the tread, the distance DE1 being greater than or equal to0.95·L, the inner boundary being formed, at least in part, by a lateralwall of a circumferential groove, the axial distance between the outerboundary and the inner boundary defining an axial width LL1 of the firstlateral rib. This axial width may vary according to the radial depth ofthe first lateral rib. The first lateral rib and at least one (andpreferably each) of the central ribs comprise a first portion made of arubber composition containing at least one elastomer and at least onereinforcing filler containing a carbon black, the carbon blackrepresenting a percentage greater than or equal to 50% and less than orequal to 100% of the weight of all the reinforcing filler, and a secondportion made of a rubber composition containing at least one elastomerand at least one reinforcing filler, possibly including a carbon black,the carbon black representing a percentage greater than or equal to 0%and less than or equal to 50% of the weight of all the reinforcingfiller. The first portion of the first lateral rib extends from theouter boundary of the first lateral rib axially inwards, the axial widthLP1 of this portion being, over the entire radial depth of the centralrib, greater than or equal to 20% of the axial width LL1 of the firstlateral rib, and the second portion of the first lateral rib is axiallyadjacent to the first portion of the first lateral rib. The firstportion of the central rib extends axially from the outer boundary ofthe central rib, the axial width LPC of this portion being, over theentire radial depth of the central rib, greater than or equal to 20% ofthe axial width LC of the central rib, and the second portion of thecentral rib is axially adjacent to the first portion of the central rib.The rubber compositions that make up the second portion of the firstlateral rib and the second portion of the central rib all have a tan d0value which is less than the tan d0 value of the rubber compositionsthat form the first portion of the first lateral rib and the firstportion of the central rib, where tan d0 denotes the value of tan dmeasured at a temperature of 0 C and under a stress of 0.7 MPa.

This tire makes it possible to obtain a marked improvement in grip onwet ground while at the same time maintaining excellent grip on dryground.

For preference, the said percentage of the carbon black in terms of theweight of all the reinforcing filler is not the same for the said firstrubber composition and for the said second rubber composition; thedifference between the percentage of carbon black in the said firstrubber composition and the percentage of carbon black in the said secondrubber composition is preferably greater than 5 points and morepreferably still, greater than 10 points.

According to a first advantageous embodiment, the axial width LP1 of thefirst portion of the first lateral rib is, over the entire radial depthof the first lateral rib, less than or equal to 60% of the axial widthLL1 of the first lateral rib. Wet grip is thus preserved.

According to a second advantageous embodiment, the axial width LPC ofthe first portion of the central rib is, over the entire radial depth ofthe central rib, less than or equal to 60% of the axial width LC of thecentral rib. This embodiment also contributes to preserving good wetgrip.

According to a third advantageous embodiment, a second lateral rib isadjacent to the inner axial edge of the tread and axially delimited byan outer boundary and an inner boundary, the inner boundary beingsituated, over the entire radial depth of the second lateral rib, anaxial distance DE2 from the outer axial edge of the tread, the distanceDE2 being greater than or equal to 0.95·L, the outer boundary beingformed, at least in part, by a lateral wall of a circumferential groove,the axial distance between the outer boundary and the inner boundarydefining an axial width LL2 of the second lateral rib. This axial widthcan vary according to the radial depth of the second lateral rib. Thesecond lateral rib comprises a first portion made of a rubbercomposition containing at least one elastomer and at least onereinforcing filler containing a carbon black, the carbon blackrepresenting a percentage greater than or equal to 50% and less than orequal to 100% of the weight of all the reinforcing filler, and a secondportion made of a rubber composition containing at least one elastomerand at least one reinforcing filler, possibly including a carbon black,the carbon black representing a percentage greater than or equal to 0%and less than or equal to 50% of the weight of all the reinforcingfiller. The first portion of the second lateral rib extends from theouter boundary of the second lateral rib axially outwards, the axialwidth LP2 of this portion being, over the entire radial depth of thecentral rib, greater than or equal to 20% of the axial width LL2 of thesecond lateral rib, and the second portion of the second lateral rib isaxially adjacent to the first portion of the second lateral rib. Therubber composition forming the second portion of the second lateral ribhas a tan d0 value which is less than the tan d0 value of the rubbercomposition that forms the first portion of the second lateral rib,where tan d0 denotes the value of tan d measured at a temperature of 0 Cand under a stress of 0.7 MPa. This embodiment makes it possible toincrease the dry grip of the tire situated on the inside of the corner;this tire thus makes a greater contribution towards the total transverseloading.

For preference, the axial width LP2 of the first portion of the secondlateral rib is less than or equal to 60% of the axial width LL2 of thesecond lateral rib. This limitation contributes to preserving the wetgrip.

According to a fourth advantageous embodiment, for each rib comprising afirst and a second portion, the difference between the tan d0 value ofthe rubber composition that forms the first portion of the rib and thetan d0 value of the rubber composition that forms the second portion ofthe same rib is greater than or equal to 0.05. As already mentioned, tand0 denotes the value of tan d measured at a temperature of 0° C. andunder a stress of 0.7 MPa.

According to a fifth advantageous embodiment, for each rib comprising afirst and a second portion, the rubber composition that forms the firstportion of the rib also has a tan d10 value which is less than the tand10 value of the rubber composition that forms the second portion of thesame rib, where tan d10 denotes the value of d measured at a temperatureof 10 C and under a stress of 0.7 MPa.

For preference, for each rib comprising a first and a second portion,the difference between the tan d10 value of the rubber composition thatforms the first portion of the rib and the tan d10 value of the rubbercomposition that forms the second portion of the same rib is greaterthan or equal to 0.05.

According to a fifth advantageous embodiment, the same rubbercomposition forms the first portion of all the ribs that have a firstand a second portion, and the same rubber composition forms the secondportion of these ribs. This embodiment has the advantage of ease ofindustrial production.

Of course, it is possible and even desirable to combine two or more ofthe embodiments described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a tire according to the prior art.

FIG. 2 is a partial perspective view of a tire according to the priorart.

FIG. 3 is a depiction, in radial section, of a quarter of a tireaccording to the prior art.

FIGS. 4 and 5 illustrate how the axial edge of a tread is determined.

FIG. 6 illustrates the terms “inner edge” and “outer edge” of a tread.

FIG. 7 schematically depicts the crown of a tire according to the priorart, in radial section.

FIGS. 8 and 13 illustrate the various measurements that characterize thelateral ribs and the central ribs.

FIGS. 9 to 12 schematically depict the crown of a tire according to oneembodiment of the invention, in radial section.

DETAILED DESCRIPTION OF THE DRAWINGS

When using the term “radial” it is appropriate to make a distinctionbetween the various uses made of this word by those skilled in the art.Firstly, the expression refers to a radius of the tire. It is in thissense that a point P1 is said to be “radially inside” a point P2 (or“radially on the inside of” the point P2) if it is closer to the axis ofrotation of the tire than is the point P2. Conversely, a point P3 issaid to be “radially outside” a point P4 (or “radially on the outsideof” the point P4) if it is further away from the axis of rotation of thetire than is the point P4. Progress will be said to be “radially inwards(or outwards)” when it is in the direction towards smaller (or larger)radii. It is this sense of the term that applies also when matters ofradial distances are being discussed.

By contrast, a thread or reinforcement is said to be “radial” when thethread or the reinforcing elements of the reinforcement make an anglegreater than or equal to 80 and less than or equal to 90 with thecircumferential direction. Let us specify that, in this document, theterm “thread” is to be understood in a very general sense and comprisesthreads in the form of monofilaments, multifilaments, chords, foldedyarns or equivalent assemblies, irrespective of the material of whichthe thread is made or of the surface treatment it has received in orderto encourage it to bond with the rubber.

Finally, a “radial section” or “radial cross section” here means asection or a cross section on a plane containing the axis of rotation ofthe tire.

An “axial” direction is a direction parallel to the axis of rotation ofthe tire. A point P5 is said to be “axially inside” a point P6 (or“axially on the inside of” the point P6) if it is closer to themid-plane of the tire than is the point P6. Conversely, a point P7 issaid to be “axially outside” a point P8 (or “axially on the outside of”the point P8) if it is further away from the mid-plane of the tire thanis the point P8. The “mid-plane” of the tire is the plane which isperpendicular to the axis of rotation of the tire and which lies equaldistances from the annular reinforcing structures of each bead.

A “circumferential” direction is a direction which is perpendicular bothto a radius of the tire and to the axial direction. A “circumferentialcross section” is a cross section on a plane perpendicular to the axisof rotation of the tire.

A “tread surface” here means all of those points of the tread of a tirewhich are able to come into contact with the ground when the tire isbeing driven on.

A rib is said to be “adjacent to the outer (or inner) edge” of the treadwhen it comprises the outer (or inner) edge or when there is no otherrib positioned axially between the said outer (or inner) edge and therib in question.

The “radial depth” of a rib is defined as the maximum depth of thegroove adjacent to the rib if the rib is delimited only by a singlegroove, as is generally the case with the lateral ribs. If the rib isaxially delimited by two grooves (as is the case with the central ribs),its radial depth is defined as the greater of the maximum depths of itstwo adjacent grooves.

The expression “rubber composition” denotes a composition of rubbercontaining at least one elastomer and one filler.

FIG. 1 schematically depicts a tire 10 according to the prior art. Thetire 10 comprises a crown comprising a crown reinforcement (not visiblein FIG. 1) surmounted by a tread 40, two side walls 30 extending thecrown radially inwards, and two beads 20 radially on the inside of theside walls 30.

FIG. 2 schematically depicts a partial perspective view of a tire 10according to the prior art and illustrates the various components of thetire. The tire 10 comprises a carcase reinforcement 60 made up ofthreads 61 coated with rubber composition and two beads 20 eachcomprising annular reinforcing structures 70 which hold the tire 10 onthe rim (not depicted). The carcase reinforcement 60 is anchored in eachof the beads 20. The tire 10 further comprises a crown reinforcementcomprising two plies 80 and 90. Each of the plies 80 and 90 isreinforced with threadlike reinforcing elements 81 and 91 which areparallel within each layer and crossed from one layer to the other,making angles of between 10 and 70 with the circumferential direction.The tire further comprises a hoop reinforcement 100, arranged radiallyon the outside of the crown reinforcement, this hoop reinforcement beingformed of reinforcing elements 101 oriented circumferentially and woundin spirals. A tread 40 is laid on the hoop reinforcement; it is thistread 40 which provides contact between the tire 10 and the road. Thetire 10 depicted is a “tubeless” tire: it has an inner liner 50 made ofa rubber composition that is not permeable to the inflating gas,covering the interior surface of the tire.

FIG. 3 schematically depicts, in radial section, one quarter of areference tire 10 of the “Pilot Sport 2” type sold by Michelin. The tire10 comprises two beads 20 intended to come into contact with themounting rim (not depicted), each bead 20 comprising a plurality ofannular reinforcing structures 70. Two side walls 30 extend the beads 20radially outwards and join together in a crown 25 comprising a crownreinforcement formed of a first layer of reinforcing elements 80 and ofa second layer of reinforcing elements 90 and radially surmounted by ahoop reinforcement 100 which is itself radially surmounted by a tread.Two grooves 141 and 142 may be seen; the incision 150 is not consideredto be a groove within the meaning of the invention because only anincision which, when the tire is in the new state, has a radial depthgreater than or equal to 5 mm and a maximum axial width greater than orequal to 2 mm is considered to be a “groove” within the meaning of thisdocument. The mid-plane of the tire is indicated by the reference 130.

The way in which the axial edges of a tread are determined isillustrated in FIGS. 4 and 5 each of which show the profile of half atread 41 and of the part of the side wall adjacent to it. In certaintire designs, the transition from the tread to the side wall is verymarked, as in the case depicted in FIG. 4, and determining the axialedge 45 of the half-tread 41 can be done intuitively. However, there aretire designs in which the transition between tread and side wall iscontinuous. An example is given in FIG. 5. The edge of the tread is thendetermined as follows. The tangent to the tread surface of the tire atany point on the tread surface in the region of transition between treadand side wall is plotted onto a radial section of the tire. The axialedge is the point at which the angle a (alpha) between the said tangentand an axial direction is equal to 30. When there are several points atwhich the angle a (alpha) between the said tangent and an axialdirection is equal to 30, the point adopted is the point that isradially furthest towards the outside. In the case of the tire depictedin FIG. 3, the axial edge 45 has been determined this way.

Each layer of reinforcing elements 80 and 90 comprises threadlikereinforcing elements coated in a matrix made of rubber composition. Thereinforcing elements of each layer are substantially parallel to oneanother; the reinforcing elements of the two layers are crossed from onelayer to another at an angle of around 20, as is well known to thoseskilled in the art of tires referred to as radial tires.

The tire 10 also comprises a carcase reinforcement 60 which extends fromthe beads 20 through the side walls 30 to the crown 25. This carcasereinforcement 60 in this instance comprises threadlike reinforcingelements oriented substantially radially, namely which make with thecircumferential direction an angle greater than or equal to 80 and lessthan or equal to 90.

The carcase reinforcement 60 comprises a plurality of carcasereinforcing elements; it is anchored in the two beads 20 between theannular reinforcing structures 70.

FIG. 7 schematically depicts the crown of a tire according to documentWO 2011/076680 A1, intended to be mounted on a mounting rim of a wheelof a vehicle and having a predetermined direction of mounting on thevehicle. It comprises a tread extended between an outer axial edge 45and an inner axial edge 46, the inner axial edge 46 being the edgeintended to be mounted on the vehicle body shell side when the tire ismounted on the vehicle in the said predetermined direction of mounting,as suggested in FIG. 6 which schematically indicates a vehicle 200. Thetread surface is indicated by the reference 47.

The tread comprises, in any radial section, three central ribs 411 to413 and two lateral ribs 421 and 422 which are positioned on each sideof the central ribs. The ribs are separated by circumferential grooveseach having two lateral walls.

FIG. 8 illustrates the various parameters that characterize the firstlateral rib 421 and the central rib 411. The central rib 411 is axiallydelimited by an outer boundary, formed by the lateral wall 1411 of thecircumferential groove 141 and situated on the side of the outer axialedge 45 of the tread, and an inner boundary, formed by the lateral wall1420 of the circumferential groove 142 and situated on the side of theinner axial edge 46 of the tread (see FIG. 7). The axial distance, onthe tread surface in the new state, between the outer boundary and theinner boundary defines the axial width LC of the central rib.

The first lateral rib 421 is adjacent to the outer axial edge 45 of thetread and axially delimited by an outer boundary FE1 and an innerboundary, which is formed by the lateral wall 1410 of thecircumferential groove 141. The outer boundary is located, over theentire radial depth of the first lateral rib 421, at an axial distanceDE1 (see FIG. 7) from the inner axial edge 46 of the tread, the distanceDE1 being greater than or equal to 0.95·L. In FIG. 7, the axial distanceDE1 of the first lateral rib 421 on the tread surface in the new stateis indicated; here DE1 is equal to 0.96·L. The axial distance betweenthe outer boundary and the inner boundary defines an axial width LL1(see FIG. 8) of the first lateral rib 421. In this particular instance,this axial width varies according to the radial depth of the firstlateral rib 421.

In the crown depicted in FIG. 7, the central rib 412 and the secondlateral rib 422 are made of a first rubber composition containing anelastomer and at least one reinforcing filler possibly including acarbon black, the carbon black representing a percentage greater than orequal to 0% and less than or equal to 50% of the weight of all thereinforcing filler.

In contrast, the first lateral rib 421 and the central ribs 411 and 413are made of a second rubber composition that contains an elastomer andat least one reinforcing filler including a carbon black, the carbonblack representing a percentage greater than or equal to 50% and lessthan or equal to 100% of the weight of all the reinforcing filler.

The first rubber composition has a tan d0 value which is lower than thetan d0 value of the second rubber composition, where tan d0 denotes thevalue of d measured at a temperature of 0 C and under a stress of 0.7MPa.

The central ribs 411 and 413 are made entirely of this second rubbercomposition; in other words, for each of these ribs, the portion madefrom this second rubber composition has an axial width equal to 100% ofthe axial width LC of the central rib.

While this solution has made it possible to achieve an excellentcompromise between wet ground and dry ground, there is still room forimprovement in terms of wet grip. Such an improvement is achieved usinga tire according to one embodiment of the invention. FIGS. 9 to 12depict embodiments of this tire.

The tire the crown of which is depicted in FIG. 9 is intended to bemounted on a mounting rim of a vehicle wheel (not depicted); it has apredetermined direction of mounting on the vehicle, comprising a treadextending between an outer axial edge 45 and an inner axial edge 46, theinner axial edge 46 being the edge intended to be mounted on the vehiclebody shell side when the tire is mounted on the vehicle in the saidpredetermined direction of mounting (see FIG. 6). The axial distancebetween the outer axial edge 45 and the inner axial edge 46 defines theaxial width L of the tread.

The tread comprises, in any radial section, three central ribs 411 to413 and two lateral ribs 421 and 422, the lateral ribs 421 and 422 beingpositioned one on each side of the central ribs 411 to 413. The ribs areseparated by circumferential grooves 141 to 144 each having two lateralwalls. Each of the central ribs 411 to 413 is axially delimited by anouter boundary and by an inner boundary, each boundary being formed,over the entire depth of the adjacent circumferential grooves (141 to143) by a lateral wall of a circumferential groove, the inner boundarybeing axially closer to the inner axial edge 46 of the tread than theouter boundary. The axial distance, on the tread surface in the newstate, between the outer boundary and the inner boundary defining anaxial width LC for each of the central ribs (see FIG. 8 in which theaxial width LC is indicated for the central rib 411). In this instance,these axial widths LC are substantially identical for the three centralribs 411 to 413, but this does not have to be the case.

The first lateral rib 421 is adjacent to the outer axial edge 45 of thetread. In this particular instance, the first lateral rib 421 does notinclude the outer axial edge 45 because it is covered at its end with aportion of rubber composition that forms the side wall of the tire, butit is nonetheless adjacent to the outer axial edge 45 in as much asthere is no other rib situated between the outer axial edge 45 and thefirst lateral rib 421. The first lateral rib 421 is delimited axially byan outer boundary FE1 (see FIG. 8) and an inner boundary whichcoincides, over the entire depth of the circumferential groove 141, withthe lateral wall 1410 of the circumferential groove 141 (see FIG. 8).The outer boundary is situated, over the entire radial depth of thefirst lateral rib, an axial distance DE1 from the inner axial edge ofthe tread, the distance DE1 being greater than or equal to 0.95·L. InFIG. 9, the axial distance DE1 of the first lateral rib 421 on the treadsurface in the new state has been indicated; here, DE1 is equal to0.96·L. The axial distance between the outer boundary and the innerboundary defines an axial width LL1 (see FIG. 8) of the first lateralrib 421. In this particular instance, this axial width varies accordingto the radial depth of the first lateral rib 421.

The first lateral rib 421 and the central ribs 411 each comprise a firstportion (4211 and 4111 respectively) made from a rubber compositioncontaining at least one elastomer and at least one reinforcing fillerincluding a carbon black, the carbon black representing a percentagegreater than or equal to 50% and less than or equal to 100% of theweight of all the reinforcing filler, and a second portion (4212 and4112 respectively) made of a rubber composition containing at least oneelastomer and at least one reinforcing filler, possibly including acarbon black, the carbon black representing a percentage greater than orequal to 0% and less than or equal to 50% of the weight of all thereinforcing filler.

The first portion 4211 of the first lateral rib 421 extends from theouter boundary FE1 (see FIG. 8) of the first lateral rib 421 axiallyinwards, the axial width LP1 (see FIG. 8) of this portion being, overthe entire radial depth of the central rib, greater than or equal to 20%of the axial width LL1 of the first lateral rib 421. In FIG. 8, theaxial width LP1 of the first portion 4211 of the first lateral rib 421on the tread surface in the new state has been indicated; here, thewidth LP1 is equal to 0.40·LL1. The second portion 4212 of the firstlateral rib 421 is axially adjacent to the first portion 4211 of thefirst lateral rib 421; it extends, over the entire depth of the firstlateral rib 421, from the end of the first portion 4211 to the innerboundary of the first lateral rib 421.

The first portion 4111 of the central rib 411 extends axially from theouter boundary of the central rib 411, the axial width LPC (see FIG. 8)of this first portion 4111 being, over the entire radial depth of thecentral rib, greater than or equal to 20% of the axial width LC of thecentral rib 411. In FIG. 8, the axial width LPC of the first portion4111 of the first lateral rib 411 on the tread surface in the new statehas been indicated; here, the width LPC is equal to 0.50·LC. The secondportion 4112 of the central rib 411 is axially adjacent to the firstportion 4111 of the central rib 411; it extends, over the entire depthof the first lateral rib 411, from the end of the first portion 4111 tothe inner boundary of the first central rib 411 (which coincides, overthe entire depth of the circumferential groove 142, with the lateralwall 1420 thereof).

The rubber compositions that make up the second portion of the firstlateral rib and the second portion of the central rib all have a tan d0value which is less than the tan d0 value of the rubber compositionsthat form the first portion of the first lateral rib and the firstportion of the central rib.

This tire according to one embodiment of the invention makes it possibleto obtain a marked improvement in wet grip while at the same timemaintaining excellent grip on dry ground. The applicant company explainsthis surprising discovery as follows: in a tire according to oneembodiment of the invention, unlike the reference tire, the outerlateral rib is made of a rubber composition that provides good grip inthe wet. This runs counter to the teaching of document WO 2011/076680according to which it is appropriate to create the outer rib from arubber composition that provides good grip on dry ground, in order toput the trapezoidal shape of the contact patch to good use. Now, drivingwith a great deal of side slip or drifting (i.e. under the conditionsencountered when driving on a circuit), a significant part of the outerlateral rib is seen to blister, which means that the rubber compositionthat confers good grip on dry ground is unable to provide all of thehoped-for benefit at this point. By arranging this rubber composition onthe outer edge of the first lateral rib and on the outer edge of (atleast) one central rib, it is possible to achieve both good grip on dryground and protection of the central ribs against premature wear.

FIG. 10 schematically depicts the crown of another tire according to oneembodiment of the invention. Unlike the tire depicted in FIG. 9, thistire comprises two central grooves 411 and 412 each having the firstportion (4111 and 4121 respectively) made of a rubber compositioncontaining at least one elastomer and at least one reinforcing fillercontaining a carbon black, the carbon black representing a percentagegreater than or equal to 50% and less than or equal to 100% of theweight of all the reinforcing filler and a second portion (4112, 4122respectively) made of a rubber composition containing at least oneelastomer and at least one reinforcing filler, possibly including acarbon black, the carbon black representing a percentage greater than orequal to 0% and less than or equal to 50% of the weight of all thereinforcing filler.

FIG. 11 schematically depicts the crown of another tire according to oneembodiment of the invention. Unlike the tire depicted in FIG. 9, thistire comprises three central grooves 411 to 413 each having a firstportion (4111, 4121 and 4131 respectively) made of a rubber compositioncontaining at least one elastomer and at least one reinforcing fillerincluding a carbon black, the carbon black representing a percentagegreater than or equal to 50% and less than or equal to 100% of theweight of all the reinforcing filler, and a second portion (4112, 4122and 4132 respectively) made of a rubber composition containing at leastone elastomer and at least one reinforcing filler, possibly including acarbon black, the carbon black representing a percentage greater than orequal to 0% and less than or equal to 50% of the weight of all thereinforcing filler.

In FIGS. 9 to 11, the central ribs comprising two portions are thecentral ribs closest to the outer axial edge 45 because it is thesecentral ribs that are more exposed to wear and providing the secondrubber composition on these central ribs has the effect of protectingthem against wear. Nonetheless, it is perfectly possible to plan foronly the central rib 413 or only the central ribs 412 and 413 tocomprise two portions, etc. For the sake of conciseness, theseembodiments which in theory are less advantageous from a wearstandpoint, have not been depicted.

FIG. 12 schematically depicts the crown of another tire according to oneembodiment of the invention; FIG. 13 shows part of this same crown. Thetire depicted comprises a second lateral rib 422, which is adjacent tothe inner axial edge 46 of the tread and axially delimited by an outerboundary which coincides, over the entire depth of the circumferentialgroove 144, with the lateral wall 1442 of this groove (see FIG. 13), andan inner boundary FI2 (see FIG. 13). The inner boundary FI2 is located,over the entire radial depth of the second lateral rib 422, an axialdistance DE2 from the outer axial edge 45 of the tread, the distance DE2being greater than or equal to 0.95·L. FIG. 13 indicates the axialdistance DE2 of the second lateral rib 422 on the tread surface in thenew state; here, DE2 is equal to 0.95·L. The axial distance between theouter boundary and the inner boundary defines an axial width LL2 (seeFIG. 13) of the second lateral rib 422. In this particular instance,this axial width varies according to the radial depth of the secondlateral rib 422.

The second lateral rib 422 comprises a first portion 4221, made of arubber composition containing at least one elastomer and at least onereinforcing filler including a carbon black, the carbon blackrepresenting a percentage greater than or equal to 50% and less than orequal to 100% of the weight of all of the reinforcing filler, and asecond portion 4222 made of a rubber composition containing at least oneelastomer and at least one reinforcing filler, possibly including acarbon black, the carbon black representing a percentage greater than orequal to 0% and less than or equal to 50% of the weight of all thereinforcing filler.

The first portion 4221 of the second lateral rib 422 extends from theouter boundary of the second lateral rib 422 axially outwards, the axialwidth LP2 of this portion being, over the entire radial depth of thecentral rib, greater than or equal to 20% of the axial width LL2 of thesecond lateral rib 422. FIG. 13 indicates the axial width LP2 of thefirst portion 4221 of the second lateral rib 422 on the tread surface inthe new state; here, the width LP2 is equal to 0.40·LL2. The secondportion 4222 of the second lateral rib 422 is axially adjacent to thefirst portion 4221 of the second lateral rib 422; it extends, over theentire depth of the second lateral rib 422, from the end of the firstportion 4221 to the inner boundary FI2 of the second lateral rib 422.

The rubber composition that makes up the second portion 4222 of thesecond lateral rib 422 has a tan d0 value which is less than the tan d0value of the rubber composition that forms the first portion 4221 of thesecond lateral rib 422.

A person skilled in the art will appreciate that the invention is not inany way restricted to a tire comprising three central ribs; it isperfectly possible to conceive of embodiments comprising one, two ormore than three central ribs.

Table I gives, by way of example, the composition of rubber compositionsthat can be used. The composition is given in PHR (“per hundred rubber”)which means in parts by weight per 100 parts by weight ofrubber/elastomer.

TABLE I Rubber composition Rubber composition forming the first formingthe second portion of the rib portion of the rib Elastomer: SBR [1] 100100 N 234 [2] 100 — Silica — 100 TESPT coupling — 8.0 agent (Si-69Degussa) Plasticizer [3] 50 50 Ozone wax C32 ST 1.5 1.5 Antioxidant(6PPD) 2.0 2.0 [4] Diphenylguanidine — 1.7 (DPG) ZnO 1.8 1.8 Stearicacid 2.0 2.0 Sulphur 1.3 1.3 Accelerator (CBS) 1.95 1.95 Notes for TableI: [1] SSBR with 40% stirene, 48% of 1-4 trans polybutadiene functionalgroups [2] Carbon black from 230 series (ASTM) [3] TDAE (treateddistillate aromatic extract) oil [4]N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine

The rubber compositions are preferably based on at least one dieneelastomer, a reinforcing filler and a crosslinking system.

A “diene” elastomer (used interchangeably with the word “rubber”) means,in the known manner, an elastomer derived at least in part (i.e. ahomopolymer or a copolymer) from diene monomers, i.e. from monomersbearing two carbon-carbon double bonds, conjugated or unconjugated. Thediene elastomer used is preferably selected from the group consisting ofpolybutadienes (BR), natural rubber (NR), synthetic polyisoprenes (IR),butadiene-stirene (SBR) copolymers, isoprene-butadiene (BIR) copolymers,isoprene-stirene (SIR) copolymers, butadiene-stirene-isoprene (SBIR)copolymers and blends of these elastomers.

One preferred embodiment involves using an “isoprene” elastomer, i.e. ahomopolymer or a copolymer of isoprene, or in other words, a dieneelastomer selected from the group consisting of natural rubber (NR),synthetic polyisoprenes (IR), the various copolymers of isoprene andblends of these elastomers.

The isoprene elastomer is preferably natural rubber or syntheticpolyisoprene of the cis-1,4 type. From among these syntheticpolyisoprenes, use is preferably made of polyisoprenes which have aproportion (molar %) of cis-1,4 bonds higher than 90%, more preferablystill, higher than 98%. According to other preferred embodiments, thediene elastomer may consist, in full or in part, of another dieneelastomer such as, for example, an SBR elastomer (E-SBR or S-SBR) usedcut or otherwise with another elastomer, for example of the BR type.

The rubber composition may also contain all or some of the additivesconventionally used in the rubber matrixes intended for tiremanufacture, such as, for example, reinforcing fillers such as carbonblack or inorganic fillers such as silica, coupling agents for inorganicfillers, anti-ageing agents, antioxidants, plasticizers or extensionoils, whether the latter be of aromatic or non-aromatic nature (notablyoils that are very weakly aromatic or non-aromatic, for example of thenaphthene or paraffin type, of high or preferably low, viscosity, MES orTDAE oils, plasticizing resins with a high TG above 30 C),processability agents for the compositions in the raw state, tackifyingresins, a cross-linking system based either on sulphur or on donors ofsulphur and/or peroxide, accelerators, vulcanization activators orretardants, antireversion agents, acceptors and donors of methylene,such as, for example, HMT (hexamethylenetetramine) or H3M(hexamethyoxymethylmelamine), reinforcing resins (such as resorcinol orbismaleimide), known adhesion-promotion systems of the metallic salttype, for example, notably salts of cobalt or of nickel.

The compositions are produced in suitable mills, using two successivepreparation phases well known to those skilled in the art: A first phaseof thermo mechanical working or kneading (the phase referred to as the“non-productive” phase) at high temperature, up to a maximum temperatureof between 110 C and 190 C, preferably between 130 C and 180 C, followedby a second phase of mechanical work (the phase referred to as the“productive” phase) up to a lower temperature, typically below 110 C,during which finishing phase the cross-linking system is incorporated.

By way of example, the non-productive phase is conducted in a singlethermomechanical step lasting a few minutes (for example between 2 and10 min), during which all the required basic ingredients and otheradditives, apart from the cross-linking or vulcanizing system areintroduced into a suitable mill such as a conventional internal mixer.After the rubber composition thus obtained has cooled, the vulcanizingsystem is then incorporated in an external mixer such as an open mill,kept at a low temperature (for example between 30 C and 100 C). The massis then mixed (productive phase) for a few minutes (for example between5 and 15 min).

Vulcanizing (or curing) can be performed in the known way at atemperature generally of between 130 C and 200 C, preferably underpressure, for a sufficient length of time which may vary for examplebetween 5 and 90 min notably dependent on the curing temperature, on thevulcanizing system adopted and on the vulcanizing dynamics of thecomposition concerned.

Table II gives the properties of the rubber compositions the compositionof which is given in table I.

TABLE II Rubber composition Rubber composition forming the first formingthe second portion of the rib portion of the rib tan d0 to 0.7 MPa 0.760.88 tan d10 to 0.7 MPa 0.69 0.58

These properties are measured on a viscoanalyzer (Metravib VA4000) inaccordance with standard ASTM D 5992-96. The response of a sample ofvulcanized composition (cylindrical test specimen 4 mm thick and 400 mm²in cross section) subjected to a simple alternating sinusoidal shearstress at a frequency of 10 Hz is recorded with a temperature sweepingbetween 0 and 100 C, under a fixed stress of 0.7 MPa, with the tan dvalue observed at 0 C and the tan d values observed at 10 C beingrecorded in particular.

It will be recalled that, as is well known to those skilled in the art,the tan d value at 0 C (“tan d0”) is indicative of the potential to gripon wet ground: the higher the tan d at 0 C, the better the grip. Thevalues of tan d at temperatures higher than 10 C are indicative of thehysteresis of the material and of the potential to grip on dry ground.

Returning to the rubber compositions the composition of which isindicated in table I, it is noticed that the composition that forms thesecond portions has a value of tan d at 0 C (under an imposed stress of0.7 MPa) that is higher by comparison with the composition that formsthe first portions, indicating that the wet grip will be better; and avalue of tan d at 10 C (“tan d10”) that is lower by comparison with thefirst composition, this indicating that the grip on dry ground will beinferior.

Tests were carried out on a BMW 330i fitted with Pilot Super Sport tiresof size 235/35 R19. A tire equipped with a tread as depicted in FIG. 9was compared against a reference tire equipped with a tread as indicatedin FIG. 7. The rubber compositions of table I were used. The tiresaccording to one embodiment of the invention were able to maintain thelap times (Charade circuit (France); circuit length: 4 km) on dryground. Braking in the wet, by contrast, was significantly improved (by2%) whereas there was no difference in braking on dry ground.

1. A tire adapted to be mounted on a mounting rim of a wheel of avehicle and having a predetermined direction of mounting on the vehicle,comprising a tread extending between an outer axial edge and an inneraxial edge, the inner axial edge being the edge intended to be mountedon the vehicle body shell side when the tire is mounted on the vehiclein the said predetermined direction of mounting, the axial distancebetween the outer axial edge and the inner axial edge defining an axialwidth L of the tread; wherein the tread comprises, in any radialsection, at least one central rib and two lateral ribs, the lateral ribsbeing positioned one on each side of the said at least one central rib,the ribs being separated by circumferential grooves each having twolateral walls; wherein each central rib is axially delimited by an outerboundary and by an inner boundary, each boundary being formed by alateral wall of a circumferential groove, the inner boundary beingaxially closer to the inner axial edge of the tread than the innerboundary, the axial distance, on the tread surface in the new state,between the outer boundary and the inner boundary defining an axialwidth LC of the central rib; wherein a first lateral rib is adjacent tothe outer axial edge of the tread and delimited axially by an outerboundary and an inner boundary, the outer boundary being situated, overthe entire radial depth of the first lateral rib, an axial distance DE1from the inner axial edge of the tread, the distance DE1 being greaterthan or equal to 0.95·L, the inner boundary being formed, at least inpart, by a lateral wall of a circumferential groove, the axial distancebetween the outer boundary and the inner boundary defining an axialwidth LL1 of the first lateral rib; wherein the first lateral rib and atleast one of the central ribs comprise a first portion made of a rubbercomposition containing at least one elastomer and at least onereinforcing filler containing a carbon black, the carbon blackrepresenting a percentage greater than or equal to 50% and less than orequal to 100% of the weight of all the reinforcing filler, and a secondportion made of a rubber composition containing at least one elastomerand at least one reinforcing filler, the carbon black representing apercentage greater than or equal to 0% and less than or equal to 50% ofthe weight of all the reinforcing filler; wherein the first portion ofthe first lateral rib extends from the outer boundary of the firstlateral rib axially inwards, the axial width LP1 of this portion being,over the entire radial depth of the central rib, greater than or equalto 20% of the axial width LL1 of the first lateral rib, and in which thesecond portion of the first lateral rib is axially adjacent to the firstportion of the first lateral rib; wherein the first portion of thecentral rib extends axially from the outer boundary of the central rib,the axial width LPC of this portion being, over the entire radial depthof the central rib, greater than or equal to 20% of the axial width LCof the central rib, and wherein the second portion of the central rib isaxially adjacent to the first portion of the central rib; wherein therubber compositions that make up the second portion of the first lateralrib and the second portion of the central rib all have a tan d0 valuewhich is less than the tan d0 value of the rubber compositions that formthe first portion of the first lateral rib and the first portion of thecentral rib, where tan d0 denotes the value of tan d measured at atemperature of 0 C and under a stress of 0.7 MPa.
 2. The tire accordingto claim 1, wherein the axial width LP1 of the first portion of thefirst lateral rib is, over the entire radial depth of the first lateralrib, less than or equal to 60% of the axial width LL1 of the firstlateral rib.
 3. The tire according to claim 1, wherein the axial widthLPC of the first portion of the central rib is, over the entire radialdepth of the central rib, less than or equal to 60% of the axial widthLC of the central rib.
 4. The tire according to claim 1, wherein asecond lateral rib is adjacent to the inner axial edge of the tread andaxially delimited by an outer boundary and an inner boundary, the innerboundary being situated, over the entire radial depth of the secondlateral rib, an axial distance DE2 from the outer axial edge of thetread, the distance DE2 being greater than or equal to 0.95·L, the outerboundary being formed, at least in part, by a lateral wall of acircumferential groove, the axial distance between the outer boundaryand the inner boundary defining an axial width LL2 of the second lateralrib; wherein the second lateral rib comprises a first portion made of arubber composition containing at least one elastomer and at least onereinforcing filler containing a carbon black, the carbon blackrepresenting a percentage greater than or equal to 50% and less than orequal to 100% of the weight of all the reinforcing filler, and a secondportion made of a rubber composition containing at least one elastomerand at least one reinforcing filler, the carbon black representing apercentage greater than or equal to 0% and less than or equal to 50% ofthe weight of all the reinforcing filler; wherein the first portion ofthe second lateral rib extends from the outer boundary of the secondlateral rib axially outwards, the axial width LP2 of this portion being,over the entire radial depth of the central rib, greater than or equalto 20% of the axial width LL2 of the second lateral rib, and in whichthe second portion of the second lateral rib is axially adjacent to thefirst portion of the second lateral rib; wherein the rubber compositionforming the second portion of the second lateral rib has a tan d0 valuewhich is less than the tan d0 value of the rubber composition that formsthe first portion of the second lateral rib.
 5. The tire according toclaim 3, wherein the axial width LP2 of the first portion of the secondlateral rib is less than or equal to 60% of the axial width LL2 of thesecond lateral rib.
 6. The tire according to claim 1, wherein, for eachrib comprising a first and a second portion, the difference between thetan d0 value of the rubber composition that forms the first portion ofthe rib and the tan d0 value of the rubber composition that forms thesecond portion of the same rib is greater than or equal to 0.05.
 7. Thetire according to claim 1, wherein, for each rib comprising a first anda second portion, the rubber composition that forms the first portion ofthe rib also has a tan d10 value which is less than the tan d10 value ofthe rubber composition that forms the second portion of the same rib,where tan d10 denotes the value of tan d measured at a temperature of 10C and under a stress of 0.7 MPa.
 8. The tire according to claim 7,wherein, for each rib comprising a first and a second portion thedifference between the tan d10 value of the rubber composition thatforms the first portion of the rib and the tan d10 value of the rubbercomposition that forms the second portion of the same rib is greaterthan or equal to 0.05.
 9. The tire according to claim 1, wherein thesame rubber composition forms the first portion of all the ribs thathave a first and a second portion, and the same rubber composition formsthe second portion of these ribs.